Automotive coatings generally comprise a separately baked electrodeposition coating (EDC) primer, a separately baked primer surfacer layer (filler layer) applied thereto and a top coat applied thereto comprising a wet-on-wet applied color- and/or special effect-imparting base coat layer and a protective, gloss-imparting clear coat layer. The total primer surfacer plus base coat layer thickness is generally 30 to 60 μm, in case of metallic color shades (color tones) more in the lower range of 30 to 45 μm.
Processes are known from WO 97/47401 and U.S. Pat. No. 5,976,343 for the production of decorative multi-layer coatings, which processes allow for the elimination of the application and separate baking of a primer surface layer which, of course, reduces coating material consumption and total layer thickness. In these processes, a multi-layer coating structure comprising a first, modified water-borne base coat, a second, unmodified water-borne base coat and a clear coat is applied by a wet-on-wet-on-wet process comprising the joint curing of these three coating layers that are applied to a baked EDC primer. In practice, these processes use two base coat layers that allow for markedly lower total layer thickness by approximately 15 to 25 μm, than that of a conventional primer surfacer and base coat. The modified water-borne base coat is produced in these processes from an unmodified water-borne base coat by mixing with an admixture component. The modified water-borne base coat replaces a conventional primer surfacer. WO 97/47401 recommends as an admixture component, the addition of polyisocyanate crosslinking agent, while U.S. Pat. No. 5,976,343 describes the addition of polyurethane resin.
A weakness of the processes known from WO 97/47401 and U.S. Pat. No. 5,976,343 is that it is not straightforwardly possible to produce multi-layer coatings in certain color shades (“problematic color shades”). The reason is UV light (UV radiation), as a constituent of natural daylight, passes through the coating layers applied to the EDC primer to the surface of the EDC primer to a noticeable extent in the absence of a primer surfacer layer and causes degradation of the EDC primer.
The color shades which are problematic with regard to the production of primer surfacer-free multi-layer coatings are those which, while (like unproblematic color shades) providing a coating which appears to an observer to be opaque, permit an inadmissibly large amount of UV light to penetrate through the multi-layer structure of clear coat, unmodified water-borne base coat and modified water-borne base coat to the surface of the EDC primer and cause long term damage to the EDC layer. Such problematic color shades are to be found both among solid color shades (plain color shades, single-tone color shades; generally independent of observation angle; pigment content without special effect pigments) and special effect color shades. Examples may, in particular, be found among water-borne base coats with dark blue solid color shades based on phthalocyanine pigments and among water-borne base coats with specific special effect color shades, for example, dark blue metallic color shades or light metallic color shades, such as, in particular, silver color shades and among water-borne base coats with specific special effect color shades containing elevated proportions of mica pigments in the pigment content. In the case of the problematic color shades, the UV light may penetrate through the multi-layer coating structure, for example, to an extent exceeding the specified UV transmission level and reaches the EDC layer. Car manufacturers' specifications state, for example, that UV transmission through the base coat layer in the area of the complete outer skin of the vehicle body should amount to less than 0.1% in the wavelength range of from 280 to 380 nm, to less than 0.5% in the wavelength range of from 380 to 400 nm and to less than 1% in the wavelength range of from 400 to 450 nm. The possible undesired long-term consequences of an inadmissible level of UV light penetration to the EDC layer are chalking of the EDC layer and delamination of the multi-layer coating over the service life of the coated substrates.
Alternatively, the modified and/or the unmodified water-borne base coat could be applied in an overall higher layer thickness sufficient to prevent to an adequate degree the access of UV light to the EDC primer. However, this would be a backward technological step in the direction of high total film thickness.
The use of UV absorbers in clear coats or base coats is known, for example, from U.S. Pat. No. 5,574,166 and WO 94/18278, and is a solution to the problem of delamination. However, UV absorbers cannot be used to a very great extent in the base coat layers and/or the clear coat layer because of the migration tendency of the UV absorbers and because of the gradual degradation of the UV absorbers, as well as for cost reasons.
Other solutions, which approach the delamination problem from the EDC side are known from EP 0 576 943, U.S. Pat. No. 6,368,719, U.S. 2003/0054193 A1 and U.S. 2003/0098238 A1. These disclose the use of EDC coating compositions which are resistant to the action of UV light due to specially selected binders or due to the addition of suitable additives. This inevitably restricts the EDC composition, such that concessions may have to be made in relation to other technological properties, such as, for example, corrosion protection.